1. Field of the Invention
The present invention relates to electronics and communications. More particularly, the present invention relates to digital video systems.
2. Description of Related Art
In video on-demand (VOD) systems, digital compressed video and audio bitstreams are stored on a server and streamed out to a user""s set top box or subscriber station. Over the course of the VOD session, many different streams are transmitted to a set top box to allow forward and rewind and other such features.
Typically, video must be streamed continuously to the set top box in order for it to operate properly, and video servers are designed to meet this requirement. However, servers are generally not able to switch between different streams at any precise point in the streams.
The resulting transitions seen by the user usually contain visual artifacts such as macroblocking, rolling, tearing, and shaking. Modifying the server to improve transitions is not an attractive option since it would add significant complexity and cost to the most expensive part of the VOD system.
One common method for handling stream transitions is to perform an operation analogous to a digital channel change by changing the packet identifier (PID) value that the set top box uses to identify the video that it should receive. While this technique may reduce or remove some of the visual artifacts, it does not eliminate all of them. Also, this method increases the delay incurred for the transition because the method requires that after the set top box is notified of the PID change, the set top box must flush its video buffer and wait while the video buffer is refilled. Also, the display to the user during the such a reset of the decoder buffer varies between set top vendors and models. Some set top boxes will freeze the last frame of the old stream while another may go to an entirely xe2x80x9cblackxe2x80x9d frame.
Transition smoothing techniques generally are a tradeoff of latency (or delay), complexity, and visual quality. In addition, there are difficulties in achieving transition smoothing (or visual improvement of stream transitions) which is independent of set top box model.
The present invention solves the above described problems and overcomes the above described disadvantages relating to video stream transitions, The present invention includes four techniques which result in the visual improvement of transitions between streams with a same packet identifier (PID).
The first technique involves pre-conditioning the content before it is stored on the server, and the others involve altering the stream after it leaves the server but before it reaches the set top box. All four techniques are applicable in the situation where the stream transition is between two streams that utilize the same PID. Retaining the same PID reduces the delay between the time the user selects an action via the remote control to the time when the new stream is displayed on the TV.
The first technique includes two methods: a method for pre-conditioning video stream data for storage; and a method for transitioning between video streams which utilizes the pre-conditioning. The method for pre-conditioning provides clock references at predetermined transition points. The method for transitioning utilizes those clock references. As a result, the transition takes less time, and visual artifacts during the transition are largely removed. The first technique may be performed in a video server for storing and serving video stream data.
The second technique involves a method for transitioning between video streams where timing information in the new stream is modified so as to be consistent with timing information in the old stream. The method results in a nearly seamless visual display during the transition. The second technique may be performed in a system comprising a server, a distribution system, a subscriber station, and a retimer in the distribution system or in the server.
The third technique involves a method for transitioning between video streams where packets in the old stream are replaced by picture repeat packets. The method results in the display of a steady xe2x80x9cfreezexe2x80x9d frame, rather than incoherent or xe2x80x9cblackxe2x80x9d frames which otherwise occur.
The fourth technique involves a method for transitioning between video streams where packets in the old stream are replaced by null packets. The method typically also results in the display of a steady xe2x80x9cfreezexe2x80x9d frame, rather than incoherent or xe2x80x9cblackxe2x80x9d frames which otherwise occur.
The third or fourth technique may be embodied in a system including a server, a distribution network, a subscriber station, and a substituter for replacing packets in the old video stream with substitution packets. For the third technique, the substitution packets comprise picture repeat packets. For the fourth technique, the substitution packets comprise null packets.
The various techniques discussed above may be combined to further improve the smoothness and quickness of the video stream transitions. For example, the first technique may be combined with either the third or fourth technique. Similarly, the second technique may be combined with either the third or fourth technique.